Explicitly set OS2Vendor to 'PfEd'

Explicitly set OS2Vendor to 'PfEd'

That's the default value that FontForge uses.  I'd like to change it,
but fontconfig makes it available to applications as "foundry" and I
somehow ended up with Emacs recording that in its configuration.  So if
I'm going to make that change I think it should go with a major version
bump.

But at least I can make sure it doesn't change unexpectedly and give
myself somewhere to make the change when it comes.